Successful semiconductor device fabrication and operation frequently requires contacting the semiconductor device with low resistance ohmic contacts. Problems often arise in attempting to fabricate and use such contacts. For example, the contacting material may form a rectifying, rather than ohmic, contact with the semiconductor material, or it may not reliably bond to the semiconductor material, and physically unreliable electrical contacts result.
Group III-V semiconductor compounds are of much interest today, and much effort has been directed toward developing reliable ohmic contacts with such compounds. Many processes for fabricating low resistance ohmic contacts to such compounds are known. These processes typically involve the deposition of one or more layers and may or may not involve one or more heat treating steps. U.S. Pat. No. 3,214,654 describes ohmic contacts to Group III-V compounds which are formed by a layer of a metal selected from the group consisting of silver, gold, ruthenium, rhodium, palladium, osmium, irridium and platinum and a layer of either nickel or cobalt. Germanium-palladium contacts to n-type Group III-V compounds are described by U.S. Pat. No. 4,011,583.
Particular interest has recently been shown in Group III-V compounds that are useful to optical devices, such as light emitting diodes, lasers and photodetectors, that operate at wavelengths longer than 1.00 micron. It should be understood that the term "light," as used in this specification, includes both the visible and the near infrared portions of the electromagnetic spectrum. Interest in devices that operate in this region has arisen primarily because the silica-based optical fiber compositions presently contemplated for optical communication systems have smaller material dispersion, as well as low loss, above 1.00 micron than they do below 1.00 micron.
One class of light emitting devices presently contemplated for such systems uses the quaternary alloy, InGaAsP, which is grown on InP. Such devices are useful between 0.95 .mu.m and 1.68 .mu.m. These light emitting devices operate at high forward current and require high quality ohmic contacts to reduce series resistance. For this class of device, as well as others, ohmic contacts in InP are necessary.
While low resistance ohmic contacts to n-type InP can now be easily fabricated, the formation of ohmic contacts to p-type InP still presents difficulties. P-type contacts to InP have been made using Zn as the acceptor. While these contacts are quite acceptable for many purposes, they have a number of drawbacks. For example, Journal of Applied Physics, 46, pp. 452-453 (1975) reports a rather high resistance, namely, 10.sup.-3 ohm.cm.sup.2, for an electroplated Au/Zn/Au metallization. Furthermore, additional problems arise when Zn is used as the acceptor because the relative volatility of Zn makes it difficult to fabricate the contact with vacuum deposition techniques. Moreover, rapid diffusion of the Zn through the InP, together with the high doping concentrations required, may cause either junction motion or long-term device reliability problems or both.
Ohmic contacts to some group III-V compounds using Be-Au metallizations, i.e., Be is used as the acceptor, are known. For example, such metallizations have been made to p-type GaP. However, formation of these ohmic contacts has required heating the GaP devices to the relatively high temperature of 600 degrees C. for approximately 5 minutes to form the ohmic contact. Alloying temperatures of 600 degrees C. cannot be used to form ohmic contacts to either InP or InP containing devices because InP begins to decompose through P outdiffusion at approximately 400 degrees.